In the opening of Episode 12 of Siren’s Kiss, Eun-hyuk catches Woo-seok sneaking around his house and promptly knocks him unconscious. A flashback reveals the realization that Woo-seok and Seol-ah shared in the previous episode. Woo-seok informs Seol-ah that Eun-hyuk’s mother worked in insurance and was the one who took out Seol-ah’s parents’ insurance policy. Seung-jae’s note to Seol-ah further reveals that it was Eun-hyuk who advised him to take out an insurance policy.
Presenting the scene, Woo-seok wakes up tied to a chair. He manages to extract the full truth from Eun-hyuk, who confesses to the killings. Woo-seok also calls Seol-ah from his smartwatch, allowing her to hear everything. Seol-ah calls Joo-young for help. Eun-hyuk loads Woo-seok with weights and takes him to the edge of a water body, instructing him to jump. Seol-ah arrives in time and begs him to stop. Despite her pleas, Eun-hyuk shoots Woo-seok, but Seol-ah steps in the way, taking the bullet. The police arrive shortly thereafter.
Seol-ah is rushed to the hospital, where doctors say she will make a full recovery. Meanwhile, Eun-hyuk is arrested. When Seol-ah wakes up, she learns that Eun-hyuk isn’t saying anything to the cops. She and Woo-seok go to the police station, where she announces that she’s going to turn herself in as the person who ordered Eun-hyuk to commit the killings. This pushes Eun-hyuk to finally confess. He reveals that he noticed Su-ho’s behavior and mannerisms in Park Jun-beom early on. The allergy trigger in the champagne was his final test, after which he went to the villa and killed Su-ho. He also admits to killing Chairwoman Kim since she wasn’t treating Seol-ah well. He says she had been financially supporting him for ten years in return for using his name for art transactions.
Woo-seok senses that there's something he's still hiding, and Seol-ah tries to confront Eun-hyuk directly, but he refuses to speak. Even the detectives feel like there's something off about his statement regarding Chairwoman Kim. Meanwhile, Woo-seok continues to take care of Seol-ah at the hospital, assuring her that they can start anew as a normal couple.
Elsewhere, Detective Pyo's wife comes to see him, knowing that he must be beating himself up for letting Hyun-su go home. After Seol-ah is discharged from the hospital, she takes Woo-seok to his sister's resting place. Later, Eun-hye comes to see her and admits that she noticed Eun-hyuk acting weird but didn't want to think the worst of him.
Eun-hyuk killed Seol-ah's boyfriends because he saw them as unworthy of her. He explains that Choi Young-ho was just a junkie who was using Seol-ah, while Seung-jae was swayed by Yun-ji's words and began to suspect Seol-ah of being after his money. This made them unworthy of her in Eun-hyuk's eyes. Yun-ji happened to see him coming out of the lake after killing Seung-jae, which is why she also had to die.
After being arrested, Eun-hyuk asks Seol-ah to visit him. He tells her about his childhood, how his father used to beat him, and how Eun-hyuk pushed him down the stairs during one of the incidents, leading to his death. Soon after, his mother took off with the death benefit from the insurance. When Seol-ah's parents committed suicide, Eun-hyuk found Seol-ah passed out in the house and set it on fire so that their sacrifice wouldn't be in vain. But he let Seol-ah believe that she did it. Chairwoman Kim saw him carrying Seol-ah out of the house and blackmailed him, threatening to tell Seol-ah his secret in order to make him do her bidding. After learning all of this, Seol-ah tells Eun-hyuk that he's the one who's caused her the most misery. Sometime later, we learn that Eun-hyuk has committed suicide.
The scene shifts to an event hosted by Woo-seok and his colleagues for little kids, where Seol-ah turns up to teach them how to draw. Detective Pyo later visits Woo-seok to tell him that Hyun-su has been given a fifteen-year sentence, apologizing for Woo-seok's long struggle.
As for Seol-ah and Woo-seok, Woo-seok comes home to find Seol-ah cooking a meal for both of them. That night, Woo-seok proposes to her but she says she's planning on studying abroad. She wants to pursue art therapy and use it to help her. He puts the ring on her finger anyway and says he can take a sabbatical and go with her. He will be by her side no matter what decision she takes.
Siren's Kiss ends with Seol-ah packing up her stuff at work and handing over her duties to Sang-mi. We then see her and Woo-seok looking at her father's painting, which now proudly hangs on the wall of a gallery.
